1. Field of the Invention
The present invention relates to a broadband wireless communication system, and in particular, to an apparatus and method for selecting a relay station in a broadband wireless communication system.
2. Description of the Related Art
In the next generation communication system, also known as the 4-th Generation (4G) communication system, research is actively in progress to provide a Quality of Service (QoS) with a data transfer speed of about 100 Mbps. In particular, research on a communication system supporting high speed service while providing mobility and ensuring QoS is actively being conducted. An example of such a communication system is the Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system.
The IEEE 802.16 communication system employs an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme allowing a physical channel of a wireless communication system to support a broadband network. In the IEEE 802.16 communication system, there is a growing demand for the mobility of a Mobile Station (MS) and the flexibility of implementing a wireless network. Further, it is also demanded that services be effectively provided in a network environment where a traffic distribution or a call request rate rapidly changes. The communication system under consideration, which is capable of dealing with these demands is one in which a multi hop relay type data transfer method is employed by the use of a relay station and an MS located near the relay station.
Throughput can be improved when the relay station is used in the broadband wireless communication system. That is, where a direct link channel between a Base Station (BS) and an MS has poor quality, the relay station may be used to provide relatively better channel quality. Furthermore, by utilizing the relay station, spatial diversity effect can be achieved since multiple-replicas of the transmitted signal are received via multiple links formed by BS, relays, and MS. In case of downlink transmission, multiple links are composed of BS-relay-MS links and a BS-MS link. Multiple links can be also formed by MS-relay-BS links and a MS-BS link in case of uplink transmission.
A method for selecting the most suitable relay station is needed for the effective use of relay stations. Conventional methods to select relay stations can be classified into two cases, selection of relays with or without channel information.
The method without channel information includes a Random Selection (RS) method and a Fixed Priority Selection (FPS) method. In the RS method, a BS randomly selects one relay station from a plurality of candidate relay stations. In the FPS method, priority of each relay station is predetermined for an MS. Since channel information is not used in the RS method and the FPS method, the BS can select relay stations with a simple procedure. However, there is no guarantee that the selected relay station is the most suitable relay station to achieve the best performance.
Meanwhile, methods with channel information include a Maximum received SNR Selection (MSS) method and a Maximum Total SNR Selection (MTS) method. In the MSS method, channel information (e.g., Signal to Noise Ratio (SNR)) between the BS and the relay station is used for the relay selection. In the MTS method, channel information between the BS and the relay station and channel information between the relay station and the MS are both used. That is, in the MSS method, the BS directly measures the channel information between the BS and the relay station, and then the relay station is selected using the channel information. In the MTS method, in addition to the channel information between the BS and the relay station, the relay station is selected by receiving the feedback of the channel information between the MS and the relay station which cannot be directly estimated at BS. By using them, BS derives total SNRs for all possible relay channels and selects relay stations which form relay channels with best total SNRs. Therefore, when using the MTS method, the selection of the most suitable relay station is ensured since all possible relay channels have been taken into account.
Accordingly, when a relay station is selected in a broadband wireless communication system, the MTS method can be a solution to select the most suitable relay station. However, when using the MTS method, overhead increases due to the feedback of channel information for relay-MS links. In addition, the process of selecting a relay station is further complicated since the total SNR of relay channel should be calculated for all possible relay channels.